Event-based Programming

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Event-based Programming

• Roger Crawfis

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Window-based programming

• Most modern desktop systems are window-based.

What location do I use to set this pixel?
Window based environment
Non-window based environment
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Event-based Programming

• Window-based GUIs are typically comprised of
  many independent processes.
• These processes sit and wait for the user to do
  something, to which it can respond.
• Moreover, a simple application may be waiting for
  any of a number of things.

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Event-based Programming

• Sample main programs
• C / managed C
• static void Main()
• Application.Run( new Form1())
• C (MFC Document/View architecture)
• There isnt one!!
• GLUT
• void main(int argc, char argv)
• myInit()
• glutMainLoop()

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Programming forms

• Procedural programming
• code is executed in sequential order.
• Event-driven programming
• code is executed upon activation of events.

statement 1 statement 2 statement 3 --------
-------- statement n
method 1 method 2 method 3 --------
-------- method n
Do method 2then method 1 then method 3
Do method 1then method 3
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Procedural programming

• Up to now, our control flow model has been pretty
  straight-forward.
• Execution starts at main() and executes statement
  sequentially branching with if,for,and while
  statement, and occasional method calls.
• When we need user input we call read() on the
  console stream which waits (blocks) until the
  user types something, then returns.
• One problem with this model is How do we wait
  for and respond to input from more than one
  source (eg keyboard and mouse). If we block on
  either waiting for input, we may miss input from
  the other.

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Event-driven programming

• the system waits for user input events, and these
  events trigger program methods
• Event-based programming addresses the two
  problems
• How to wait on multiple input sources (input
  events)
• How to decide what to do on each type of input
  event

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General form of event-driven programming

• The operating system and window system process
  input events from devices (mouse movement, mouse
  clicks, key presses etc)
• The window system decides which window, and hence
  which frame and program, each input event belongs
  to.

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General form of event-driven programming

• A data structure describing the event is produced
  and placed on an event queue. Events are added to
  one end of the queue by the window system. Events
  are removed from the queue and processed by the
  program. These event data structures hold
  information including
• Which of the program's windows this event belongs
  to.
• The type of event (mouse click, key press, etc.)
• Any event-specific data (mouse position, key
  code, etc.)

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Event loop

• main()
• ...set up application data structures...
• ...set up GUI....
• // enter event loop
• while(true)
• Event e get_event()
• process_event(e) // event dispatch routine
  
• the event loop is usually hidden from programmer,
  he/she provides just a process_event() method

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AWT Applications - Frame

• Frame is a container for components

Frame with normal window controls
Optional Menu
Three containers in Frame with Border Layout
UI-components inside containers each with own
layout
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Understanding the GUI

• UI-containers
• have list of UI-components
• Each UI-component
• is a class
• with paint method
• lists of Event listeners

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Understanding .NET UI Components

• Overview
• Replacement for VB Forms and MFC
• Fully integrated with the .NET framework
• Web Service aware
• Data (ADO.NET and XML) aware
• Secure code and Licensing
• A framework for building Windows application
• No touch deployment
• No registration, Versionable, Smart caching
• A RAD environment in Visual Studio .NET
• Windows Form Designer, Property Browser

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Overview (cont)

• Rich set of controls
• Standard controls - Label, Button, TextBox, etc.
• Common controls - ProgressBar, StatusBar, etc.
• Data aware controls - ListBox, DataGrid, etc.
• Common dialogs
• ActiveX Interoperability
• Can host ActiveX controls / author ActiveX
  controls
• Printing Support
• Visual inheritance
• Create a form based on another form by using
  inheritance
• Advanced layout
• Anchoring and docking

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Basic Terms

• Component
• Timer, DB Connection
• Control (User Control)
• Windows Forms
• Menu
• Button
• etc
• Web Forms
• ASP.NET specific

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Using VS.NET

• Adding Components and Controls
• Just drag component from the toolbox
• Editing properties
• Edit directly in the properties editor
• Registering for events
• Also in the properties editor

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What actually happens?

• Data members for components
• For example, after placing timer
• public class FunnyButton System.Windows.Forms.
  Button
• private System.Windows.Forms.Timer timer1

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What actually happens?

• Attributes for the properties
• For example, after changing Text property
• private void InitializeComponent()
• this.Text "ADO Example"

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What actually happens?

• Delegates for events
• Special type event in .NET (special delegate)
• private void InitializeComponent()
• this-gtMouseEnter new System.EventHandler(this.
  UserControl1_MouseEnter)
• private void UserControl1_MouseEnter(object
  sender, System.EventArgs e)

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Names everyone should know

• Properties
• Name data member name
• Text text shown by control (was Caption)
• ForeColor current text color
• Dock docking to the parent
• Enabled active or not
• Events
• Click mouse click on the control
• SizeChanged - resize

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InitializeComponent() method

• This method is created by VS.NET
• Code generated there represents all the changes
  done in the design view
• Note if you remove event handler or data member
  added by VS.NET manually, do not forget to clean
  the code that uses it from InitializeComponent().
  
• Doing this from the design view is easier!